The utilization of ceramic fibers for insulation of high temperature furnaces has been practiced in several methods. In one method, sheets of ceramic fiber are attached to the wall much in the manner of layered wallpaper. It is known in such constructions to utilize lower temperature resistant ceramic fibers toward the cold face of the shell of the furnace with higher temperature resistant fibers forming the layers on the hot face. This system has disadvantages in that it is difficult to install and requires numerous studs which must be carefully positioned and themselves must be temperature resistant. Further, the studs conduct heat to the cold face thereby decreasing the efficiency of the insulation.
It has also been practiced to utilize modules of ceramic fiber which are installed as units with the fibers of the modules arranged such that most of the fibers in the ceramic fiber mats lay in places generally perpendicular to the wall. U.S. Pat. No. 3,819,468 to Sauder illustrates such a module system. These module systems may be installed by screwing, bolting or welding the modules to the furnace shell. The modules have enjoyed success because of the ease of installation, efficiency of the heat insulation and ease of replacement of damaged modules. However, the edge grain module system has suffered from the disadvantage that only one type of ceramic fiber may be utilized in a module. Therefore, expensive high temperature ceramic fibers must be utilized to form the entire module whereas the fibers near the cold face are exposed to temperatures much lower than their maximum use temperature.
There also have been developed very high temperature ceramic fibers formed by solution gelling techniques which are commonly referred to as polycrystalline fibers. Such fibers are illustrated by U.S. Pat. Nos. 3,996,145 to Hepburn, 3,322,865 to Blaze, Jr., 4,277,269 to Sweeting and 4,159,205 to Miyahara. These polycrystalline fibers have a working temperature up to about 3000.degree. F. However, these fibers are very expensive and expense has limited their use. It also has been a practice in the industry to blend the polycrystalline fibers with lower temperature ceramic fibers to create a higher temperature resistant material with better strength properties and also higher temperature resistance.
It has been proposed to utilize polycrystalline very high temperature fibers in modules. Such modules are ordinarily only about 3 inches in thickness and are cemented over hard refractories or cemented to ceramic fiber modules. Such installation techniques are expensive in use of fiber and further are subject to failure as the cement bond between the hard refractory and the fibers or between the two types of fiber is subject to failure upon cycling of the furnaces and further is very dependent upon high quality, careful installation.
Modules of composite construction have been proposed. Such modules have a lower temperature fiber forming the interior of a cushion-like block. The outer covering of the cushion being a higher temperature ceramic fiber material. However, it is not believed that polycrystalline ceramic fiber insulating materials have been successfully used in such constructions as mats of polycrystalline fibers have been too weak when used as a surface covering. They have lacked both strength and abrasion resistance to articles in the furnace and even strong air currents in furnaces. Polycrystalline fiber structures of sufficient strength and abrasion resistance have been too rigid to be bent around the sides of the cushion and be compressed during installation. Modules formed of rigid board material are difficult to install without leaving gaps. Further, modules of a construction using several different types of fiber have been found to be expensive in labor costs for construction and in materials costs for fastening of the various elements of the composite together.
Therefore, there remains a need for a system of presenting the very high temperature resistent fibers to the hot face of a furnace wall using only the minimum amount of these fibers at the hot face. Further, there remains a need for such a system that will allow utilization in module constuction or in other constructions which place the edge grain of the module towards the hot face with the fibers of the mat laying generally perpendicular to the wall or ceiling of the furnace. There is a further need for a commercially feasible system of providing insulation to furnaces operating with hot face temperatures of about 3000.degree. F.